CRON
cron is a time-based scheduling service in Unix-like computer operating systems. The name is derived from Greek chronos (χρόνος), meaning time. cron has been recreated several times in its history.

Greylisting
Greylisting is a simple method of defending electronic mail users against e-mail spam. In short, a mail transfer agent which uses greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried, however, even spam sources which re-transmit later will be more likely to be listed in DNSBLs and distributed signature systems such as Vipul's Razor.

MTA
A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon)), is a computer program or software agent that transfers electronic mail messages from one computer to another. The term mail server is also used to mean a computer acting as an MTA that is running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name System formally refers to an IP address assigned to a device hosting a mail server, and by extension also indicates the server itself.

SMTP
Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.